If it is desired to measure the thickness of thin layers on small articles, it is necessary to know, still more accurately, where the X-rays strike the small article, for it is certainly not intended to perform the measurement on the article as a whole. The typical size of the spot on which the measurement is performed varies between a few tenths of a millimeter, up to one millimeter. Small measurement-spots are necessary because knowledge is desired, for example, of the thickness of the layer, precisely at a specified point on the flank of a gearwheel tooth, or of the thickness of the layer in the contact region of a contact tongue, etc.
It is, of course, impossible to see where the X-ray beam strikes the layer.
Of one apparatus which is on the market, it is known, imprecisely, that the surface to be measured can somehow be observed, by means of a mirror, on the one hand, and that the mirror is then pushed away, to be replaced by the irradiating device.